M.Sc Student | Denneboom Dror |
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Subject | Modeling of Phosphorous adsorption by Aluminum water treatment residuals |

Department | Department of Civil and Environmental Engineering |

Supervisors | Professor Emeritus Abraham Shaviv |

Professor Michael Iggy Litaor | |

Full Thesis text |

Aluminum water treatment residuals (Al-WTR) are a by-product of water treatment processes in water treatment plants worldwide and are composed of amorphous aluminum hydroxides, which show a strong affinity to the phosphate ion. In this research, a model for the flow of a phosphorous (P) solution through a cylindrical column of Al-WTR was developed based on the continuum approach, using a one dimensional advection - dispersion - reaction equation with a bilinear adsorption rate term. Model parameters such as bulk density, effective porosity, maximal adsorption capacity, dispersivity and kinetic adsorption coefficients were measured experimentally for a sample of Al-WTR taken from a local water treatment facility. Experiments of P flow through the Al-WTR column were conducted for inlet P concentrations of 5 mg/L and 1000 mg/L. The P breakthrough curves measured in the experiments were compared with the results of the model that were obtained by a numerical simulation using the finite difference method. The model results showed poor agreement with the low concentration experiment, with a negative value, showing an under-estimation of the adsorption capacity. For the high concentration experiment, results of the model were better fitted with , but with an apparent over-estimation of the initial process of adsorption which may be due to pH effects, not taken into account in the model. In both cases, the model results did not capture the long-term adsorption of P in the Al-WTR column. It was decided to remodel the adsorption as consisting of two separate processes - a process of surface adsorption, and a process of intra-particle diffusion and adsorption to micropore walls within the particles. This approach is equivalent to the known mobile - immobile formulation with two kinetic sites by Van Genuchten. A mathematical description of the diffusion process was developed based on the intra-particle diffusion equation, and a numerical solution was developed and incorporated into the original model. Diffusion model parameters such as diffusivity coefficient, particle porosity, particle density and micropore P adsorption capacity were calculated approximately based on previous work on intra-particle diffusion in Al-WTR done by Makris et al. Very good agreement with experimental results was shown using this model for the low-concentration experiment, with , whereas the results of the high-concentration experiment where only slightly improved with . It was shown that the intra-particle diffusion model captures the slow adsorption process that controls the break-through curve behavior after the initial stage of surface adsorption, and therefore improves the predictive ability of the numerical simulation. Sensitivity analysis was performed using the high concentration experiment as a test case, and it was shown that the model is most sensitive to uncertainty in the bulk density and the maximal adsorption capacity of the Al-WTR, and shows low sensitivity to the uncertainty in the variables used in the diffusion model.